I don't think you need to be terribly concerned as the buyer about the prohibition on selling user names -- that's more there to deter people from cybersquatting (and the resultant extortion to transfer ownership) than it is to prevent a legitimate business from claiming a name that is related to it. Transferring the name shouldn't be difficult except for the fact that anyone who approaches the current owner is going to be asked the obvious question -- i.e., why do you want the name? If the twitter name is closely associated with your company, and you are the logical buyer, then protecting your anonymity is probably not realistic. And if you have a lawyer make the initial approach I'm afraid you may not not actually be sending the "I want it at the best price" signal to the current owner, who will likely see dollar signs when counsel comes calling with a request to buy the twitter name.
But Twitter's terms do allow a "separate legal agreement" for sale of usernames, so get an IP lawyer to contact their support arm to see what's involved in a username purchase.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
As you wisely recognize, you should retain counsel to guide you through this. As an attorney, I would only be willing to assist you if you agree to handle everything in a legal and ethical manner, which means full disclosure to both (a) Twitter, and (b) the buyer of (1) who you are and why you want the name, and (2) why the price you are paying is fair in the circumstances. To the extent you want to conduct yourself in a manner that hides relevant facts (especially facts concerning why you might be willing to pay good money for the name), you can later be accused of fraud (i.e., misrepresenting or omitting material facts). I could not begin to evaluate the situation until I learned about your business and the seller's reasons why he or she might want to hold onto the name. The key concept here is fairness--you need to do everything in the open, with full disclosure, and you must expect to pay a fair and reasoanble price. And guess what---the seller has no obligation to sell and Twitter has no obligation to consent to the sale. Finally, you should prepare yourself for the distinct possibilty that you might not get what you want. Surely there must be an alternative Twitter user name that would work for you in the event you cannot buy the one you want.